Fix import error for default material

[philsson]

This error appears during initial import in the AssetPostprocessorHandler.OnPostprocessPrefab callback, which runs during asset import. Shader.Find() attempts to find the CrossRPDefault shader which is not yet loaded. This causes the Material constructor to throw an ArgumentNullException.

This is a non-issue past the first import so this fix it merely to suppress the error

ArgumentNullException: Value cannot be null.
Parameter name: shader
UnityEngine.Bindings.ThrowHelper.ThrowArgumentNullException (System.Object obj, System.String parameterName) (at <1b6535aa091d41cf8b3ca222ed9ceef0>:0)
UnityEngine.Material.CreateWithShader (UnityEngine.Material self, UnityEngine.Shader shader) (at <1b6535aa091d41cf8b3ca222ed9ceef0>:0)
UnityEngine.Material..ctor (UnityEngine.Shader shader) (at <1b6535aa091d41cf8b3ca222ed9ceef0>:0)
AGXUnity.Utils.RenderingUtils.CreateDefaultMaterial () (at Assets/AGXUnity/AGXUnity/Utils/RenderingUtils.cs:129)
AGXUnity.Rendering.WireRenderer.DefaultMaterial () (at Assets/AGXUnity/AGXUnity/Rendering/WireRenderer.cs:109)
AGXUnityEditor.AssetPostprocessorHandler.OnPostprocessPrefab (UnityEngine.GameObject gameObject) (at Assets/AGXUnity/Editor/AGXUnityEditor/AssetPostprocessorHandler.cs:65)
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <41d29b352f6a475ab1bf7c6628b82790>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <41d29b352f6a475ab1bf7c6628b82790>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <41d29b352f6a475ab1bf7c6628b82790>:0)
UnityEditor.AssetPostprocessingInternal.InvokeMethodIfAvailable (System.Object target, System.String methodName, System.Object[] args) (at <2e4eaf245b924ed1b8c17467295ae042>:0)
UnityEditor.AssetPostprocessingInternal.CallPostProcessMethods (System.String methodName, System.Object[] args) (at <2e4eaf245b924ed1b8c17467295ae042>:0)
UnityEditor.AssetPostprocessingInternal.PostprocessPrefab (UnityEngine.GameObject prefabAssetRoot) (at <2e4eaf245b924ed1b8c17467295ae042>:0)